In a fuel injection system of a compression-ignition internal combustion engine, it is known to use a so-called ‘distributor pump’ to deliver pressurised fuel to a series of fuel injectors. The distributor pump serves the dual functions of i) pressurising the fuel to a desired injection pressure and ii) delivering a charge of pressurised fuel to each of the fuel injectors at the exact moment it is required to inject fuel into a corresponding combustion cylinder. In some vehicle applications, distributor pumps are favoured since they achieve a cost reduction over so-called ‘in line’ pumps that comprise a cam-driven pumping plunger unit for each fuel injector of the engine.
FIG. 1 shows a schematic view of a typical distributor-type fuel pump 2, the functionality of which would be well known to the skilled reader. The fuel pump 2 comprises a pump body or casing (not shown) within which a longitudinally extending pump shaft 4 is rotatably mounted. In use, the pump shaft 4 is driven by the drive shaft of an associated engine and so the speed of rotation of the pump shaft 4 is proportional to the speed of the engine.
The fuel pump 2 is functionally separated into a pump drive section, indicated generally as 6, which performs the role of fuel pressurisation, and a pump distributor section indicated generally as 8, which performs the role of distributing the pressurised fuel to each fuel injector.
Referring firstly to the pump drive section 6 (shown to the left in FIG. 1), the drive shaft 4 is provided with a passage 10 extending radially therethrough within which is received a pair of diametrically opposed pumping plungers 12. The pumping plungers 12 are moveable within the radial passage 10 and define a pumping chamber 14 between their opposing faces.
The pumping plungers 12 are operable to reciprocate within the radial passage 10 by way of a cam arrangement 16 which is driven by a cam ring 18. The cam arrangement 16 includes first and second cam shoes 20 that engage a respective one of the pumping plungers 12 at its radially outer end. The cam shoes 20 are shaped to receive a respective cam roller 22 in such a manner that the cam roller 22 is free to rotate in the cam shoe 20.
Although it is not clear in FIG. 1, the cam ring 16 is of annular form and its cam surface is shaped so that as the drive shaft 4 rotates, the cam rollers 22 are caused to ride over the cam surface and move radially to impart a synchronised reciprocating motion to the pumping plungers 12. Thus, as the pump shaft 4 rotates, the pumping plungers 12 are caused to move inwardly together to perform a pumping stroke, in order to force pressurised fuel out of the pumping chamber 14, and outwardly together to perform a filling stroke, in order to suck fuel into the pumping chamber 14.
Fuel flows to and from the pumping chamber 14 via a passage 24 provided in the pump shaft that communicates with the pump chamber 14 and extends longitudinally along the axis of the pump shaft 4 into the distributor section 8 of the fuel pump 2. The longitudinal fuel passage 24 conveys fuel at an injectable pressure level away from the pumping chamber 14 to the distribution section 8 when the pumping plungers 12 are performing a pumping stroke and conveys fuel at a relatively low pressure (transfer pressure) to the pumping chamber 14 from the distribution section 8 when the pumping plungers 12 are performing a filling stroke.
The end of the longitudinal fuel passage 24 located in the region of the distributor section 8 is shaped so as to turn through 90 degrees and extend radially through the pump shaft 4 to terminate at its outer surface.
The distributor section 8 includes a generally cylindrical distributor head 30 within which the pump shaft 4 is rotatable such that the distributor head 30 remains stationary relative to the pump shaft 4. The distributor head 30 is provided with a one or more distributor ports 32 (only two of which are shown, with dashed lines, in FIG. 1), the number of which corresponds to the number of injectors of the engine, typically four, six or eight. The distributor ports 32 are radially spaced around the distributor head 30 and are communicable with the passage 24 in the pump shaft 4 at discrete intervals as the pump shaft 4 rotates. In use, as the pump shaft 4 rotates so as to cause the pumping plungers 12 to perform a pumping stroke, the passage 24 moves into registration with one of the distributor ports 32. Pressurised fuel will thus be communicated to the injector that is fluidly connected to said port 32. The passage 24 will register with the other distributor ports 32 in synchronisation with the pumping strokes performed by the pumping plungers 12.
The distributor head 30 is also provided with an inlet port 34 that extends radially so as to define an opening on the outer and inner faces of the distributor head 30. Although not shown in FIG. 1, the inlet port 34 is supplied relatively low pressure fuel from a fuel transfer pump (not shown) and is communicable with a cross bore 36 provided in the drive shaft 4 that intersects the longitudinal passage 24.
As the drive shaft 4 rotates, the inlet passage 34 registers with the cross bore 36 at discrete intervals as the pumping plungers 20 perform a filling stroke. As a result, fuel is drawn from the inlet passage 34, through the cross bore 36 and longitudinal passage 24, and into the pumping chamber 14, ready for the commencement of a pumping stroke.
It is critical that the rotational timing of a distributor pump is set up correctly when the pump is installed on the engine for the first time to ensure that pressurised fuel is delivered to each cylinder of the engine at the correct moment. Similarly, it is important that a distributor pump can be disconnected from and re-connected to the engine, during maintenance for example, without adversely affecting the pump timing and, thus, performance.
It is common for a prototype sample of a fuel pump 2 to be connected to a test engine following manufacture so that the performance of the fuel pump 2 can be analysed to determined the ‘correct timing position’ for that specific pump. For the purposes of this specification, the correct timing position refers to the precise angular position of the pump shaft 4 that is required to deliver fuel to the cylinder that is first in the engine firing sequence (for example, No. 1 cylinder) with the cylinder piston in the top dead centre position (TDC).
Conventionally, a ‘timing master pump’ is created during development of a pump type for an engine. The purpose of the timing master pump is to calibrate a ‘timing angle function’ of a calibration test machine. Following the manufacture of a production-standard pump, the pump is calibrated on such a test machine or ‘test bench’, typically being electric-motor driven, to determine the correct timing position for accurate fuel delivery, and to determine the desired settings for other devices such as, for example, the engine speed governor and advance box. At the end of calibration the test bench rotates the drive shaft of the pump to the correct timing position referenced from the timing master pump and further angular rotation of the pump shaft 4 is prevented by a locking bolt (not shown in FIG. 1) that is screwed into the body of the fuel pump. Once the correct timing position has been set and the locking bolt screwed in position, the pump is suitable for delivery to an engine manufacturer.
Although the above method is adequate for setting up the correct timing position of a fuel pump prior to installation of the pump on an engine, once the fuel pump has been installed, and the lock bolt released (as required for the engine to operate), the correct timing position is lost. The Applicant has recognised that a problem exists if an engine is observed as running poorly following installation of the fuel pump as there is no means to determine whether or not the initial fuel pump timing set-up is at fault. Furthermore, if the fuel pump is removed from the engine, for maintenance purposes for example, the fuel pump cannot be correctly reinstalled since the original timing position of the drive shaft is lost. In these circumstances, the fuel pump must be returned to the manufacturer for re-calibration. This is a hindrance to the pump manufacturer and the engine manufacturer since it introduces inefficiencies, and hence cost disadvantages, into production and service procedures.
Thus, an object of the invention is to enable the correct timing position of a fuel pump to be measured accurately and reliably following pump calibration and which allows the pump shaft position to be checked and reset after the pump has been connected to the engine.